Conventional fermented milks such as yoghurt or various lactic acid beverages are prepared by adding, for example, sugar, isomerized sugar or glucose to milk, skim milk powder or whey optionally together with a shape retaining agent such as gelatin or a paste such as agar.
A fermented milk is prepared by inoculating a pasteurized yoghurt mix with a starter, which is a mixture of lactic acid bacteria, for example, mixture of Lactobacillus bulgaricus and Streptococcus thermophillus, effecting lactic fermentation at a definite temperature for a definite period of time until the aimed pH value is achieved and then cooling the fermented mixture to thereby cease the fermentation. The fermented milk products thus obtained are put into refrigerated storage and distributed as such. However these products are frequently stored at a temperature of 10.degree. to 15.degree. C. or higher during the distribution. Then the lactic fermentation further proceeds with the lapse of time, which lowers the pH value of the product and elevates the sourness thereof. For example, when a fermented milk showing a pH value of 4.2 immediately after the preparation is stored at a temperature of 10.degree. to 15.degree. C. for one week, the pH value thereof falls to 3.98 to 3.91. Thus the product becomes extremely sour, which considerably lowers the preference thereof.